Mira

MIRA (Omicron Ceti). Mira, its very name telling us that we should take strong notice, Mira "the amazing one," the word coming from the same root as "miracle," Mira the only proper-named star in the sky that for a time is too faint to be seen with the naked eye. Few of the stars in its resident constellation Cetus, the Whale, are prominent; only Deneb Kaitos (Beta Ceti) is of second magnitude. Mira itself was relegated by Johannes Bayer to be the "Omicron" (the 15th letter in the Greek alphabet) star. In 1572, Tycho Brahe studied a "new star" in Cassiopeia so bright that for a time it was visible in daylight. Not new at all, Tycho's Star is now known to have been an old star that exploded and disappeared forever. Two dozen years later, David Fabricius thought he might have found another, though much fainter one, in Cetus. This star, however, returned, and has been doing so for over 400 years. Mira, a class M7 red giant 420 light years away, is the brightest and nearest of the red class M "long period variables," thousands of which are known. The star varies from about third magnitude (though sometimes it can reach second) all the way down to tenth, 40 or so times fainter than the human eye can see alone, and then back again over a 330 day period (the spectral class varying as well between M5 and M9 coolest when faintest). As a result it is sometimes a part of its constellation, sometimes not. The temperatures and radii of such stars are highly problematic. They depend on where the star is in its variation cycle and on the wavelength ("color") at which they are observed. If we observe at a wavelength at which the star's outer gases are very opaque (the result of molecular absorption of light), we will find a radius that might be twice as high (and the temperature lower) than it appears at a "transparent" wavelength. The calculation of luminosity is then confused as well. Mira, however, is so close that we can easily measure its angular radius, hence physical radius, which ranges from about 2 Astronomical Units (500 solar radii) at visual wavelengths to double that in the infrared, or from 20 percent bigger than the orbit of Mars to nearly half the size of the orbit of Jupiter. From a typical temperature (found from the angular size) of around 3000 Kelvin (other estimates going as low as 2000 Kelvin) and the radius itself, we find a luminosity of around 8500 times that of the Sun, close to that deduced from the visual brightness, distance, and an estimate of the huge amount of infrared radiation produced by the cool "surface." The star is approaching the last stages of its life. Long ago, the hydrogen fusion that powered its core ran out, and then the by-product of that fusion, helium, fused to carbon and oxygen, and now the helium has also run out. The result of these internal changes is a hugely distended, very luminous star. The light variations are caused by pulsation, changes in size that also affect the star's temperature and thus the amount of light that leaks out at visual wavelengths (the infrared variation nowhere near so large). Mira's great size and instability promote a dusty wind that blows at a rate of about a tenth of a millionth of a solar mass per year (10 million times that of the solar wind) that will soon evaporate away its outer envelope to produce an ephemeral planetary nebula (such as the Ring or Saturn nebulae), the inner nuclear burning portions of the star eventually condensing into a burnt-out white dwarf, a tiny star the size of Earth, the rest of the star lost to interstellar space. These long period variables help enrich the interstellar gases, out of which new stars condense, with dust and chemical elements formed in their nuclear cauldrons. Most of the carbon in the Universe seems to have come from them. Mira has a white dwarf companion that orbits some 65 AU away from Mira proper to which all these events have already happened. (Many billions of years from now, the same will happen to our Sun.) The two are close enough that the white dwarf draws mass from Mira's wind. Massive white dwarfs that are closer to their mass-donating companions can overload themselves so much that they can collapse and explode as supernovae. Such an event is believed to have created Tycho's Star, and we are back full circle. As Mira moves through the gases of interstellar space, it leaves a shocked wake more than 15 light years long .

Written by Jim Kaler 10/23/98; revised 02/13/09. Return to STARS.